Power semiconductor modules typically included a molded plastic body such as a plastic molding compound in which power semiconductor dies are embedded, or a plastic housing which covers the power semiconductor dies. Commonly used package materials are epoxies used in transfer molding devices or as potting material in modules, silicone gel used as potting material in modules, and engineering plastics such as PBT (polybutylene terephthalate), PET (polyethylene terephthalate), polyamide, PPS (polyphenylene sulfide), etc. as housing/envelope materials. Such materials not only have to fulfil predetermined mechanical requirements (e.g. elongation at yield ≥1%, preferred; electrical RTI≥140° C., mechanical RTI≥140° C.), but also environmental (e.g. related standards: RoHS; REACH), fire and smoke (e.g. related standards: UL94V0; EN45545, including HL2 and R22) as well as electrical (e.g. related standards: IEC60664, including CTI>400, 600) and chemical (e.g. low content of solvable ions) requirements.
Mechanical requirements are a consequence of joining the plastic material to metals, substrates and/or semiconductors. The plastic material becomes elongated, bent or compressed during temperature cycling as the CTE (coefficient of thermal expansion) of the different materials does not match, and also the temperature is not homogenous if temperature cycles are generated by power losses in the power semiconductors and a temperature gradient exists between the heat sources and the environment and heatsinks. The plastic material should not crack during such stress. Therefore, the elongation at yield is an important figure. High temperature applied to the plastic also should not change the mechanical and electrical characteristics too much, which is covered by RTI (relative thermal index). RTI indicates the ability of a material to retain a particular property (physical, electrical, etc.).
Electrical and chemical requirements are combined for harsh environmental conditions. Under humid atmosphere and applied voltage bias, the plastic materials should not cause corrosion of terminals, substrates or semiconductors. An 85° C./85% RH (relative humidity) test with applied blocking voltage (e.g. 100%, 80% or 60% of blocking voltage) of the semiconductors should for example last for 168 h, 500 h or even 1000 h without causing excessive leakage current between terminals, substrate tracks or semiconductor electrodes. The REACH regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals) promulgated by the EU outlines a directive for restricting the use of hazardous substances in electrical and electronic equipment—the so-called RoHS (restriction of hazardous substances) directive (see http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2011:174:0088:0110:de:PDF)
To fulfil mechanical, environmental and fire and smoke requirements, additives are typically used. Additives such as flame retardants, fibers, minerals and other chemicals often comprise the electrical characteristics of the package. For example, dielectric strength of the molded plastic body may be weakened over a long lifetime by ions or interfaces introduced by additives. Insulating characteristics may be weakened especially in a humid environment, or in a salt mist or acidic gaseous atmosphere. Also, the package terminals may corrode where contacted by the molded plastic body and corrosive by-products may propagate over the surface of insulating portions. Semiconductor characteristics may be influenced as well by ionic additives. The predetermined electrical requirement CTI (comparative tracking index), which relates to creepage between life parts, typically requires another kind of additive.
As such, there is a need for a power semiconductor module which satisfies a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements but without adversely affecting various characteristics of the package components.